Activities of the Constituents in Spinel Solid Solution and Free Energies of Formation of MgO, MgO·Al₂O₃

Abstract

Spinel (MgO·Al₂O₃) is known as one of the most harmful non-metallic inclusions in steel. However, the technology to avoid spinel formation has not yet been established due to lack of thermodynamic information on the spinel itself. In the present work, the free energies of formation of MgO and MgO·Al₂O₃, and the activities of the constituents in spinel solid solution have been directly measured by the chemical equilibrium technique. The free energies of formation of MgO and MgO·Al₂O₃ determined in the present work are expressed by the following equation:

Mg(l) + 1/2O₂(g) = MgO(s)    ΔG°_{f, MgO} = -669350 + 135.0T   (J/mol)
Al₂O₃(s) + MgO(s) = MgO·Al₂O₃(s)     ΔG°_{f, MgO· Al2O3} = -20790 - 15.7T   (J/mol)

The activity of stoichiometric MgO·Al₂O₃ exhibits negative deviation from ideality in the spinel solid solution. Phase stability diagram of spinel formation in liquid steel was assessed as a function of aluminum, magnesium and oxygen contents in iron at 1 873 K. The predicted phase boundaries of MgO·Al₂O₃, MgO and Al₂O₃ agreed well with previous observations by one of the present authors.